Tetrafluoroethylene (TFE)/hexafluoropropylene (HFP) copolymer has superior heat resistance, chemical resistance, extrusion moldability and the like, and in addition, has a superior electric insulating property and high-frequency property with a low dielectric tangent. Therefore, it is used for insulating cable such as a cable and a wire, and such insulated cable is suitably used as a communication cable. The communication cable includes a data transmission cable such as a LAN cable.
TFE/HFP copolymer also has low flammability and low smoking properties. Thus, insulated cable made from such a copolymer can be used as a plenum cable, which is laid, for example, on the back of a ceiling of a building (plenum area) and strictly regulated for preventing the spread of fire.
The insulated cable comprises a core wire such as a cable and an insulating material formed from a resin such as a TFE/HFP copolymer coating the core wire. In general, the insulated cable is manufactured by extrusion coating in which molten resin is extruded in the shape of a tube, drawn down by inserting a core wire through the center portion of the resin tube in its axial direction, and the core wire coated with the resin is then taken up.
The term “draw-down” as used herein means extending a molten resin extruded from a die having an opening of relatively large sectional area to its final intended dimensions. The draw-down is characterized by a draw-down ratio (DDR), which is the ratio of the sectional area of the opening of the die to the sectional area of the insulated material of the final product. In general, the draw-down ratio is suitably from 50 to 150. Because draw-down extends the resin in the above-described manner, elongation melt breakage (cone-breaks) which are discontinuous portions generated in the insulating material, tend to occur.
The term “insulated” cable as used herein means a cable or wire coated with an insulating material.
In recent years, an increase in molding speed has been desired to enhance productivity and to reduce cost, and there is a demand to increase the speed at which the insulated core wire is taken up to thereby increase the coating rate. When the coating rate is increased, the insulating material thus obtained generally tends to suffer from cone-breaks as a result of draw-down even if the draw-down ratio is the same as used at lower coating rates. Moreover, adhesion to the core wire is lowered.
A TFE/HFP copolymer which can withstand an increased coating rate is in demand. Although the TFE/HFP copolymer is manufactured, e.g., by water-soluble emulsion polymerization or the like, the polymer thus obtained has a functional group such as a carboxyl group originating mainly from a reaction initiating agent at the dyads or ends of the main chain thereof. Thus, a resulting problem is that the polymer generates foaming in a high-temperature atmosphere such as during melt processing to thereby cause cone-breaks.
In order to avoid this problem, conventional TFE/HFP copolymers are generally subjected to end stabilizing treatment. The end stabilizing treatment includes changing an unstable group at the end to a stable group such as a difluoromethyl group or eliminating an unstable group by applying high temperature and/or high shearing force after polymerization as disclosed, for example, in U.S. Pat. No. 4,626,587, Japanese Kokai Publication Hei-10-80917 and the like.
The TFE/HFP copolymer to which such end stabilizing treatment has been completely applied has a low volatile content. However, if it is used for the coating extrusion, adhesion with the core wire is inferior, and in particular, a copolymer having a completely fluorinated and group causes severe shrink-back. Such a molding method is disclosed in WO 00/44797 and the like which comprises using a TFE/HFP copolymer having a melt flow rate (MFR) of 24 (g/10 min.) and a coating speed set at less than 2,000 feet/min. However, since the coating speed has further increased in recent years, the problem of shrink-back may have worsened.
Because a conventional TFE/HFP copolymer is subjected to high temperature and high shearing force at the end stabilizing treatment, the molecular weight distribution is generally narrow. If such a conventional TFE/HFP copolymer is used for the coating extrusion and if an attempt is made to increase the coating speed, stable molding can be carried out at a low to medium speed, however, when the speed exceeds a certain extent, cone-breaks can suddenly occur.
For increasing the critical speed below which cone-breaks do not occur, for example, Japanese Kokoku Publication Hei-2-7963 discloses a process for increasing melt tension by extending the molecular weight distribution. In the embodiment of this process, however, one having low coating speed, and a TFE/HFP copolymer having an MRF of 14 (g/10 min.) or less is disclosed.
Die swell is considered to be an index for the molecular weight distribution. In order to prevent cone-breaks by increasing the draw-down rate during coating extrusion, the die swell preferably has a large value. In a conventional TFE/HFP copolymer, however, because the die swell ordinarily decreases remarkably during end stabilizing treatment or the pelletizing step to cause melt fracture, high-speed coating extrusion is difficult.
Although a process of coating extrusion of a cable with a TFE/HFP copolymer having a certain die swell is disclosed in WO 01/36504, this process specifies, as the TFE/HFP copolymer, a powder obtained by polymerization. In order to increase the speed of the coating extrusion, it is generally preferred to reduce the melt viscosity of the resin. On the other hand, resistance to stress cracking of the resin decreases because of the lowered melt viscosity.
U.S. Pat. No. 4,029,868 and Japanese Kokoku Publication Sho-63-2281 propose to carry out copolymerization using perfluoro(alkyl vinyl ether) (PFAVE) as a third monomer component of a TFE/HFP copolymer in order to provide resistance to stress cracking.
U.S. Pat. Nos. 5,677,404 and 5,703,185 disclose a TFE/HFP copolymer obtained by carrying out copolymerization using PFAVE as a third component. These patent publications disclose that a PEVE-based TFE/HFP copolymer comprising perfluoro (ethyl vinyl ether) (PEVE) is superior in MIT bending life as compared to a PPVE-based TFE/HFP copolymer comprising perfluoro(propyl vinyl ether) (PPVE) as PFAVE, hence it is possible to increase the coating speed.
However, no mention is made to the adhesive strength of the insulating material to the core wire, or to cone-breaks.